Refactor core-ui

+ Remove doom/switch-theme (replaced with an advice for load-theme)
+ Reorganize core-ui
+ Fix Emacs 26+ hl-line hack being applied for Emacs 25 users
+ Rename doom|show-whitespace-maybe to doom|highlight-non-default-indentation
+ Disable tool-bar, menu-bar and vertical-scroll-bars via
  default-frame-alist; this is a little faster than using the minor
  modes.

core/autoload/ui.el

@@ -113,23 +113,6 @@ presentations."
     (doom|init-theme)
     (doom|init-fonts)))
 
-;;;###autoload
-(defun doom/switch-theme (theme)
-  "Like `load-theme', but will unload currently loaded themes before switching
-to a new one."
-  (interactive
-   (list (completing-read
-          "Load theme: "
-          (mapcar #'symbol-name
-                  (custom-available-themes)))))
-  (condition-case nil
-      (progn
-        (mapc #'disable-theme custom-enabled-themes)
-        (load-theme (intern theme) t)
-        (when (fboundp 'powerline-reset)
-          (powerline-reset)))
-    (error "Problem loading theme %s" x)))
-
 ;;;###autoload
 (defun doom*recenter (&rest _)
   (recenter))